In radiosurgery or radiotherapy (collectively referred to as radiation treatment), very intense and precisely collimated doses of radiation are delivered to the target region in the body of a patient in order to treat or destroy lesions. Typically, the target region is comprised of a volume of tumorous tissue. Radiation treatment requires an accurate spatial localization of the targeted lesions. Stereotactic radiosurgery (SRS) is a specific type of image-based treatment, which delivers a high dose of radiation during a single session. Because a single radiosurgery dose is more damaging than multiple fractionated doses, the target area must be precisely located.
In general, radiation treatments consist of several phases. First, a precise three-dimensional (3D) map of the anatomical structures in the area of interest (head, body, etc.) is constructed using any one of (or combinations thereof) a computed tomography (CT), cone-beam computed tomography (CBCT), magnetic resonance imaging (MRI), positron emission tomography (PET), 3D rotational angiography (3DRA), ultrasound techniques, single photon emission tomography (SPECT), or biplanar digital subtraction angiography (DSA). This determines the exact coordinates of the target within the anatomical structure, namely, locates the tumor or abnormality within the body and defines its exact shape and size. Second, a motion path for the radiation beam is computed to deliver a dose distribution that the surgeon and/or radiation oncologist finds acceptable, taking into account a variety of medical constraints. During this phase, a team of specialists develop a treatment plan using special computer software to optimally irradiate the tumor and minimize dose to the surrounding normal tissue by designing beams of radiation to converge on the target area from different angles and planes. Third, the radiation treatment plan is executed. During this phase, the radiation dose is delivered to the patient according to the prescribed treatment plan. The imaging modalities in each of these steps arc configured to operate within prescribed modes of operation for each type of scan performed.
Generally, quality assurance (QA) and verification protocols are instituted for each stage of the radiation treatment process. The performance of the respective radiation treatment devices, their generated images, and the transfer of those images across digital networks are calibrated and tested by phantom assemblies and devices which, when imaged by the respective imaging modality, generate images that are representative, familiar, and logical to the structure and configuration of the phantom. Systematic testing and measurement of the images should produce measurement values that fall within the range of expected and legally acceptable values which indicate that the imaging device operates within normal or acceptable levels of performance.
Existing verification phantoms include CT phantoms, slab geometry phantoms, and anthromorphic phantoms. CT phantoms are used for checking the CT number relative electron density (RED) conversion, the radiation beam geometry assessments, the digitally reconstructed radiograph (DRR) generation, and multiplanar reconstruction. Slab geometry phantoms are used for film dosimetry and corrections for inhomogeneous geometries. Anthromorphic phantoms are used for dosimetric measurements of typical or special treatment techniques. Each of these phantoms is designed to fulfill a particular verification function at a particular stage of the treatment process.
Currently there is no single universal verification phantom available that can provide end-to-end verification, and which can be simultaneously used with a range of imaging modalities to facilitate image based positioning and monitoring, as well as dosimetric analysis of the delivered dose distribution.